oSIST prEN 16211:2011

SLOVENSKI STANDARD
01-marec-2011
3UH]UDþHYDQMHVWDYE0HULWYHSUHWRND]UDNDYVLVWHPXYHQWLODFLMH0HWRGH
Ventilation for buildings - Measurement of air flows on site - methods
Lüftung von Gebäuden - Luftvolumenstrommessung in Lüftungssystemen - Verfahren
Systèmes de ventilation pour les bâtiments - Mesurages de débit d'air dans les systèmes
de ventilation - Méthodes
Ta slovenski standard je istoveten z: prEN 16211
ICS:
91.140.30 3UH]UDþHYDOQLLQNOLPDWVNL Ventilation and air-
VLVWHPL conditioning
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
DRAFT
NORME EUROPÉENNE
EUROPÄISCHE NORM
December 2010
ICS 17.120.10; 91.140.30
English Version
Ventilation for buildings - Measurement of air flows on site -
methods
Systèmes de ventilation pour les bâtiments - Mesurages de Lüftung von Gebäuden - Luftvolumenstrommessung in
débit d'air dans les systèmes de ventilation - Méthodes Lüftungssystemen - Verfahren
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 156.

If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations which
stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other language
made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2010 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 16211:2010: E
worldwide for CEN national Members.

Contents Page
Introduction . 4
Foreword . 5
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 6
4 Symbols (and abbreviated terms) . 8
5 Preparation of measurement . 9
5.1 Factors influencing measurements. 9
5.2 Sources of errors and uncertainties . 10
5.2.1 Gross errors . 10
5.2.2 Systematic errors . 10
5.2.3 Calibration . 11
5.2.4 Uncertainties . 12
5.3 Measurement requirement . 12
5.3.1 Measurements using a manometer . 12
5.3.2 Measurements using an anemometer . 13
5.3.3 Measurements using Pitot static tube . 13
5.3.4 Mean value calculation of measurement signal . 13
6 Measurement uncertainty . 13
6.1 Standard instrument uncertainty, u . 13
6.2 Standard method uncertainty, u . 14
6.3 Standard reading uncertainty, u . 14
6.4 Expanded measurement uncertainty, U . 14
m
7 Methods for measurement of air flows in ducts – ID-methods . 15
7.1 Point velocity measurements using a Pitot static tube – method ID 1 . 15
7.1.1 Method description . 15
7.1.2 Preparations to be made at the site of measurement . 17
7.1.3 Measurement procedure . 21
7.1.4 Corrections of measured values . 22
7.1.5 Standard method uncertainty . 24
7.2 Point velocity measurements using a hot wire anemometer – method ID 2 . 24
7.2.1 M ethod . 24
7.2.2 Equipment . 24
7.2.3 Measuring procedure . 24
7.2.4 Correction of measured values . 24
7.2.5 Standard Method uncertainty . 25
7.3 Fixed devices for flow measurement – method ID 3 . 25
7.3.1 Method description . 25
7.3.2 Preparations of mesurements . 26
7.3.3 Measurement procedure . 26
7.3.4 Correction of measured values . 26
7.3.5 Standard method uncertainty . 26
7.4 Tracer gas measurement – method ID 4 . 27
7.4.1 Method description . 27
7.4.2 Equipment . 28
7.4.3 Calculation of air flow . 29
7.4.4 Standard measurement uncertainty . 30
7.4.5 Conditions for homogeneous mixing of tracer gas . 30
8 Methods for Supply ATDs (air terminal devices) – ST-methods . 32
8.1 Measurement of reference pressure – method ST 1 . 33
8.1.1 Equipment . 34
8.1.2 Correction of measured values . 34
8.1.3 Standard method uncertainty . 35
8.2 The Bag Method – method ST 2 . 35
8.2.1 Limitations . 36
8.2.2 Equipment . 36
8.2.3 Preparation . 36
8.2.4 Measurement . 36
8.2.5 Correction of measured values . 36
8.2.6 Standard method uncertainty . 36
8.3 Measurements with flow hood – method ST 3 . 37
8.3.1 Introduction . 37
8.3.2 Equipment . 37
8.3.3 Correction of measured values . 39
8.3.4 Standard method uncertainty . 40
9 Methods for Extract ATDs (air terminal devices) – ET-methods . 40
9.1 Measurement of reference pressure at extract ATD – method ET 1 . 41
9.1.1 Limitations . 42
9.1.2 Equipment . 42
9.1.3 Correction of measured values . 42
9.1.4 Standard method uncertainty . 42
9.2 Measurement using a flow hood – method ET 2 . 43
9.2.1 Introduction . 43
9.2.2 Equipment . 44
9.2.3 Measurements . 45
9.2.4 Correction of measured values . 45
9.2.5 Standard method uncertainty . 46
Annex A (normative) Uncertainties . 47
Bibliography . 50

Introduction
The construction, function and maintenance of ventilation installations are of great importance for the perception of the
interior climate of a building by those who work or live there. Since it is already clear that many buildings today have
problems with their interior climate and air quality, it is essential to use reliable methods to check that the installation is
functioning as intended. The result of an inspection can have large financial implications if the installation is not
passed. It is thus vital that the inspector bases his judgement on measurement methods which are reliable and have
small, known measurement uncertainties. The construction, function and maintenance of the installation also have a
large impact on the annual running costs of the plant. It is for this reason necessary to pay close attention to these
functions of the installation.
Measurement methods which are both correct and easy to use are developed and standardised to enable the
commissioning and operational monitoring of air processing installations. Interior climate and air quality can often be
improved considerably if the heating and ventilation system is managed in a way that ensures good functioning in the
long term. It is thus important that the installation is designed at the planning stage to allow measurement and
monitoring to be performed using established and approved methods.

Foreword
This document (prEN 16211:2010) has been prepared by Technical Committee CEN/TC 156 “Ventilation
for buildings”, the secretariat of which is held by BSI.
This document is currently submitted to the CEN Enquiry.
This document is currently submitted to fill with appropriate information.
Measurement methods which are both correct and easy to use are developed and standardised to enable
the commissioning and operational monitoring of air processing installations. Interior climate and air quality
can often be improved considerably if the heating and ventilation system is managed in a way that ensures
good functioning in the long term. It is thus important that the installation is designed at the planning stage
to allow measurement and monitoring to be performed using established and approved methods.

1 Scope
This standard applies to measurement of airflows on site. It provides the technician with a description of
the methods, their protocols, and tables for noting measured and calculated values so that the necessary
measurements are performed within the margins of stipulated method uncertainties.
Note : The duct traverse method in this standard is an alternative method to the duct traverse method of
ISO 3966 and EN12599. It defines errors due to the simplified approach and describes also other methods
of measurements.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments)

EN12599 Ventilation for buildings - Test procedures and measuring methods for handing over
installed ventilation and air conditioning systems
EN14277 Ventilation for buildings – Air terminal devices – Methods for airflow measurement by
calibrated sensors in or close to ATD/Plenum boxes
ISO 3966, Measurement of fluid flow in closed conduits. Velocity area method using Pitot static tubes.
ISO 5167-1, Measurement of fluid flow by means of pressure differential devices. Part 1: Orifice plates,
nozzles and Venturi tubes inserted in circular cross-section conduits running full.
ISO 5167-2    Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full-Part 2: Orifice plates
ISO 5167-3    Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full-Part 3: Nozzles and Venturi nozzles
ISO 5167-4    Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full-Part 4: Venturi tubes
ISO 5221, Air distribution and air diffusion. Rules to methods of measuring air flow rate in an air-
handling duct.
ISO 4053-1, Measurement of gas flow in conduits -- Tracer methods -- Part 1: General
VIM :/ JCGM see reference – International Vocabulary of Metrology – basic and general concepts and
rd
associated terms (VIM°) – 3 edition, Jan 2008-04-03
ENV 13005 Guide to the expression of uncertainty of measurement
3 Terms and definitions
For the purposes of this European Standard, the following terms and definitions apply:
3.1 Hydraulic diameter
The hydraulic diameter is the diameter of a circular duct which would have the same linear pressure drop, and is
defined by the following formula:
D = 4 ⋅ A/O m   eq.3-2
h
where
A = the cross sectional area of the duct, m
O = the circumference of the duct or perimeter, m

For a rectangular duct this becomes:
D = 2 ⋅ L1 ⋅ L2 / (L1+ L2) m  eq. 3-3
h
where L1 and L2 are the sides of the duct.

For a circular duct this becomes:
D = D m   eq. 3-4
h
where
D = duct diameter, m
3.2 Flow disturbances
Flow disturbances result in velocity profiles in ducts that are non symmetrical.
Note : flow seldom has a symmetrical appearance except after long straight sections. The symmetry is often disturbed
by varying resistance, for example after a bend, an area decrease or an area increase. The velocity profile will also
become disturbed by a damper, T-piece as well as before and after a fan.
3.3 Air density, ρ
The density of dry air varies with air pressure and temperature according to the following formula:
B 273
ρ =1,293⋅ ⋅ kg/m eq. 3-5
1013 273 +ϑ
where
B = barometric pressure, h Pa (hPa). (Normal air pressure is 1013 hPa)
ϑ= air temperature, °C
Note: The relative humidity of the air (RH) has very little influence on the density of air at room
temperature. The density of air at 20°C and 1013 hPa which is saturated with water vapour is only approx.
1% less than equivalent dry air.
In a low-pressure system it is hardly necessary to consider the influence of static pressure on air
density. In a high-pressure system, however, it can be necessary. The calculation is then performed as
follows:
B +0,01⋅ p 273
s
ρ =1,293⋅ ⋅ eq. 3-6
1013 273 +ϑ
where
p = static overpressure in the duct, Pa
s
ϑ = air temperature in °C
3.4 Dynamic pressure, p
d
ρ ⋅u
p = Pa  eq. 3-7
d
where
ρ = air density, kg/m
u = air velocity, m/s
3.5 Corrections for air density, ρ
When presenting a measured flow or velocity, it should be stated if it is real flow or standardised flow that is
presented. Below in this section it is described how to convert between standardised and real velocity. The
same conversion is valid for air flow.
The method describes how to compensate and correct the air flow to real flow, that is real flow (real
velocity). The volume of air is as it is at present temperature and barometric pressure. Real flow is not the
same as standardised flow. Standardised flow is used to present the air flow by recalculating the air
volume into standard condition of 1013 hPa and 20°C (68°F). Standardised flow can directly be interpreted
as a mass flow. The reason to use real flow is that the fan approximately transports the same real air flow
irrespective of the air density. The amount of standardised flow will change with air density. Even though
the dimensioning air flow is made by using ρ = 1.20 kg/m at the fan inlet, the measured velocity is
comparable if the flow is presented as real flow. Providing that the other parameters are identical,
correction of real flow is only necessary if the density at the time of measuring, ρ , deviates from the
m
density at the fan intake, ρ .
f
No corrections for air density are required during the proportional balancing of terminals and branch ducts
providing that the entire installation is balanced under the same running conditions. For this reason,
heaters in terminals and branch ducts, for example, must be switched off.
The instrument in use may measure real or standardised flow or the instrument requires calibration
conditions to display correctly. Compensate accordingly, especially when used at other conditions than
calibration condition or standard conditions of 1013 hPa and 20°C. The barometric pressure will decrease
with altitude and also vary with weather. Temperature can also vary especially when the house is not
conditioned.
To calculate from measured (real) velocity or air flow to standard velocity or air flow use the following
formula:
v = v ⋅ ρ / ρ m/s  eq. 3-8
s m m s
where:
v = standard velocity, m/s
s
v = measured (real) velocity, m/s
m
ρ = density at time of measurement, kg/m
m
ρ = standard densityρ = 1,2 kg/m
s
Note that heating or cooling devices in the duct between the fan and the place of measurement must be
switched off while measuring air flow, q or air velocity, v .
m m
4 Symbols (and abbreviated terms)
The following symbols are used in the report.

Symbol Description SI Unit Symbol Description SI Unit
t Time s n Revolutions r.p.m.
O
α Quotient of measured and - Perimeter m
planned air flow
P Power W
Efficiency -
η
3 p
Pressure Pa
kg/m
Density
ρ
p Dynamic pressure Pa
d
p
p Planned pressure Pa
p Static pressure
s Pa
m
A Area p Total pressure Pa
t
m
a,b p Measured pressure
Dimensions of length, e.g. u Pa

height of a duct
∆p Differential pressure Pa
hPa
B
Barometric pressure
Differential commissioning
∆p Pa
ppm i
C Contaminant concentration pressure

ppm
Air flow /s, l/s
Initial concentration q m
ppm
C
i 3
Tracer gas concentration in Corrected air flow m /s, l/s
q
k
C
s stationary condition
m /s, l/s
q Planned air flow
m p
Diameter
Trace gas flow m /s, l/s
q
m s
D
Hydraulic diameter
m /s, l/s
q Total air flow
- t
D
h
Flow factor
Measured air flow m /s, l/s
q
mm u
k
Smaller dimension of a
Temperature °C
mm ϑ
L rectangular duct
Temperature
T K
mm
Larger dimension of a
rectangular duct
Temperature difference
∆ϑ, ∆T °C, K
L
Std Instrument uncertainty
Volume
m
V
u
Std Method uncertainty
Air velocity
m/s
v
u
Std Reading uncertainty
Air velocity in duct centre
m/s
v
c
u
Standard measurement
Corrected velocity
m/s
vk
u
m
uncertainty
Air velocity, mean value
m/s
v
m
U
m
Expanded measurement
uncertainty
5 Preparation of measurement
5.1 Factors influencing measurements
The measurement methods which are to be used must have known and small method uncertainties. The requirements
for method uncertainties must be to some extent related to the requirement for flow tolerances.
There are many factors which affect the measurement results and which must be checked in connection with
measuring. Examples of these are:

‰ Calibration equipment, which must be regularly compared with a traceable norm (calibration unit).
‰ Calibrated measurement instruments.
‰ Calibration intervals.
‰ Examination of instruments’ long term stability.
‰ Instruments’ temperature or density compensation.
‰ Random instrument uncertainties.
‰ Random reading uncertainties.
‰ Variations in the measured quantity.
‰ Measurement methods adapted to different installation cases.
‰ Deviations when measuring from calibrated installation cases.
‰ Random uncertainties in measurement methods.
‰ Measurement methods’ influence on the flow rate.
‰ Variations in the exterior climate.
‰ Air flow stability.
5.2 Sources of errors and uncertainties
The result of numerical work is influenced by many types of error. Certain sources of error are difficult to
manipulate, others can be reduced or even eliminated.
Errors in given data input may be the result of measurements which have been affected by system errors
or temporary disturbances. Errors in measurement data can be divided into:
• Gross errors
• Systematic errors
• Uncertainties
5.2.1 Gross errors
Gross errors should not be allowed but they may happen by accident as a result of the human factor. The risk of
gross errors arising can be reduced by suitable design of working
conditions and working routines. Stress, tiredness and poor lighting are common reasons for gross errors.
Checks should be planned, either of the end result before it is approved or of the work in
progress, to prevent large amounts of work being repeated as a result of an early error.
Incorrect magnitude of measured values or insufficient regularity can often be discovered
at an early stage. It is also possible to make a check on the likelihood of many results by
examining associations between them. Time taken on designing working conditions and
verification measures is time well spent.

5.2.2 Systematic errors
According to the definition, systematic errors occur if the individual measurement values deviate in the same direction
from the “true” value or if they vary in a regular fashion.
The result of measurements where systematic errors occur may appear as in Figure 1.

Key
1 Measured
2 True value
3 Systematic error
X Time
Figure 1: Explanation of systematic error

The circles represent measured numbers which lie randomly spread around the true value and which according to the
definition are thus free from systematic errors.
The crosses represent results of measurements where the measured numbers lie too high, for example as a result of
an uncalibrated measuring instrument being used. This error can easily be rectified by calibrating the instrument and
determining a correction.
The following applies to a correction:
Correction = (true value) – (read value)
or
(Read value) + (correction) = estimate of true value

Estimates of true values are also often called measured values.

5.2.3 Calibration
Calibration is a part of the determination of the systematic errors of an instrument, which allows the understanding of
the calibration uncertainty, to eventually set up the instrument or correct the measurements, and by its repetition to
assess the drift uncertainty.
An instrument must always be able to give a correct measured value. This means that calibration must take place at
regular time intervals. It is recommended that electronic instruments used for pressure, flow and velocity
measurements are calibrated regularly according to their drift to obtain the uncertainty required. The instrument and
other equipment that influence the measurement result (ex. Bag in bag method)should be calibrated using a method
with a (known) low uncertainty, traceable to international calibration standards.
Calibration tables where corrections, or alternatively the real value, are evident should be used.
The measured value is obtained by correcting the read-out value according to:

Measured value = (read-out value) + (correction for the instrument)

The correction should therefore be stated as an absolute value, not as a factor, considering that corrections will be
made in the field
5.2.4 Uncertainties
Even if systematic errors are successfully eliminated, repeated measurements of the same quantity may not produce
identical results despite the measurements being made thoroughly. This type of error is usually defined as a result of
chance and is called uncertainty. This means that the size and character of the uncertainty cannot be shown to follow
any known law and it can therefore not be calculated or corrected for in advance. These uncertainties are normally
assumed to be composed of a number of small instrument uncertainties together with rapid and uncontrolled variations
in environmental conditions.
Temporary (random) uncertainties are divided into instrument uncertainties, method uncertainties and reading
uncertainties, and are discussed in more detail in section 6

5.3 Measurement requirement
A measurement must be based on a well-defined method, in which both measurement points as well as the measuring
instrument must be decided. This does not mean that certain selected instruments should be standardised, but that
there is a decided procedure with norms for the instrument to be used.
Measurement values are evaluated in a specified way for the method chosen, after which the values are corrected for
the method. A correction factor is commonly used at this point, from which:

.
Correct value = (measured value) (correction factor for the method)

5.3.1 Measurements using a manometer
When measuring with a manometer, digital display instruments with a resolution according to table 1 should be used.
Table 1 −−−− Resolution for the ranges of manometers
Range Resolution
Pa Pa
Up to and including 50 0.1
Above 50 1
The lowest acceptable pressure measurement is 3 Pa when mean values are taken over at least 1.5 second. If only
single measurements are made the lowest acceptable measured pressure is 5 Pa.
The manometer should be zeroed before each measurement or the manometer should be equipped with a function
which automatically zeros the instrument after a certain time period or before each measurement or compensates the
measurement with the offset checked with a following auto zero.

5.3.2 Measurements using an anemometer
Hot wire anemometers should not be used for measuring velocities less than 0.5 m/s when the air flow is to be
determined. If a hot wire anemometer is used at lower velocity, the total uncertainty (in %) will be increased due to the
influence of the hot wire absolute uncertainty.
Mechanical anemometers should not be used when measuring velocities less than 1 m/s If a vane anemometer is used
at lower velocity (in close range of the start threshold of the device), the total uncertainty (in %) will be increased due to
the influence of the vane anemometer absolute uncertainty.
The overall diameter of the device obstructing the duct passage area should not exceed 1/10 of the duct diameter.

5.3.3 Measurements using Pitot static tube
The diameter of the Pitot static tube obstructing the duct passage area should not exceed 1/30 of the
diameter
Measurement using a Pitot static tube should not be used for making velocity measurements of less than 2.5 – 3 m/s
Note : it is linked to the minimum acceptable pressure measurement (see 5.3.1)

5.3.4 Mean value calculation of measurement signal
In order to eliminate reading uncertainties as much as possible, instruments with a mean value calculation function
should be used.
6 Measurement uncertainty
It is important that when calculating uncertainties, u, using eq. 6-1 they shall all have the same coverage probability of
approximately 68%. [5]
The measurement standard uncertainty, u , is calculated using the following formula:
m
2 2 2 ½
u = (u + u + u )  eq. 6-1
m 1 2 3
where u , u and u are random standard uncertainties with a coverage probability of approximately 68%.
1 2 3
u = standard instrument uncertainty,
u = standard method uncertainty, , resulting from deviations from the calibration method for the measurement method.
In this type are also included deviations from the calibration curve for series-produced measurement devices, dampers
or terminals with in-built measurement outlets. The method uncertainty is normal distributed.
u = standard reading uncertainty, % The reading uncertainty is rectangular distributed for digital instruments.
6.1 Standard instrument uncertainty, u
Even after correcting a read value or a measured mean value with regards to different factors, there still remain
random uncertainties in measurements. Instrument uncertainty includes calibration uncertainty and uncertainty from
the instrument itself (such as hysteresis, temperature compensation, drift…)
Information on this uncertainty must be supplied by the instrument manufacturer and it is important to check that the
coverage probability of approximately 68% is used.
Some instruments have an upper and lower uncertainty value (limit) and the uncertainty can in this case be judged to
value
be rectangular distributed.  u =
Note: Corrections are known errors and not included in the instrument uncertainty. Correct the measurement values by
using corrections from the calibration certificate.

6.2 Standard method uncertainty, u
When measurements are taken, an accurately specified method should be used. As a result of deviations from the
method, e.g. the orientation of a probe, distance between the probe and a grille etc, certain random uncertainties will
be produced by the method. For those methods described in chapters 7 – 10, the uncertainty is stated as a standard
uncertainty with 68% coverage probability – one standard deviation for each method. The method uncertainties are
normal distributed.
6.3 Standard reading uncertainty, u
This type of uncertainty can be attributed to reading uncertainties, so that the resolution may play a large part,
especially with analog instruments.
For digital instruments, reading uncertainty, u = of resolution.
2 3
In case of digital pulse readout the uncertainty has to be estimated or an average function over time can be used for
certain instruments. For instruments with an analog display, the standard uncertainty can be estimated as 1/6 of a
scale interval (u ).
3s
6.4 Expanded measurement uncertainty, U
m
To cover most measurement results it is recommended that the coverage probability of approximately 95%
is used for presenting the final measurement uncertainty. The standard uncertainty of measurement is
stated for a normal distribution as the standard uncertainty of measurement multiplied by the coverage
factor k = 2. That means that the measurement uncertainties will cover approximately 95% of the
measurements and that 5% will fall outside the stated uncertainties.
U = ku , where k = 2
m m
7 Methods for measurement of air flows in ducts – ID-methods
Overview of recommended methods :
Method DesignationSee Standard method uncertainty, u
paragraph
Point measurements using a ID 1 7.1 Criteria: v>2,5-3 m/s
Prandtl (Pitot-static) tube:
1. Circular cross-section ID 11 4% (case A) – 6% (case B)

ID 12
2. Rectangular cross-section 4%
Point measurement using hot- ID 2 7.2
wire anemometer or
mechanical anemometer::
ID 21 4% (case A) – 6% (case B)
1. Circular cross-section
ID 22
4%
2. Rectangular cross-section
Fixed flow measurement ID 3 7.3 5%
devices
(Applies generally using the installation
ID 31
1. Without damper measurements obtained with testing
method)
ID 32
2. With damper
Tracer gas ID 4 7.4 5% or 10% depending on mixing length

7.1 Point velocity measurements using a Pitot static tube – method ID 1
7.1.1 Method description
This method involves the flow being calculated from a series of velocity measurements in the duct cross-
section. The determination of velocity is carried out using a Pitot static tube, by which the velocity is
calculated from dynamic pressure definition.
A Pitot static tube measures both total and static pressure. It consists in principle of a tube placed inside
another tube. Both the static and the dynamic pressure enter the orifice of the inner tube. In order for the
measurement result to be correct, the orifice must point directly into the axis of flow. The static pressure is
measured through a hole in the mantle surface of the outer tube. The principles involved in making
measurements with a Pitot static tube are illustrated in Figure 2.

Key
1 The connection for static pressure
2 The connection for total pressure

Figure 2: Measurements with Pitot static tubes

The following relationship may be described for the pressure conditions at the connections for total and
static pressure:
Connection for total pressure:
ρ ⋅v
p = p + p = p + eq. 7-1
s d s
Connection for static pressure:
p = p  eq. 7-2
s
If we now connect both pressure outlet points to a manometer, the two static pressures which exist at both
connections will even each other out and we will obtain a reading equivalent to:
ρ ⋅v
p =  eq. 7-3
d
The air velocity can be determined from this dynamic pressure:
2 ⋅ p
d
v =  eq. 7-4
ρ
7.1.2 Preparations to be made at the site of measurement
7.1.2.1 Equipment
• Pitot static tube with indicated insertion length
• Manometer including plastic tubes, thermometer and barometer
7.1.2.2 Necessary straight sections before and after plane of measurement
The flow profile has a distorted appearance after certain disturbances to the flow, e.g. bends or dampers. If
measurement takes place directly after a flow disturbance there is a risk of poor accuracy. For this reason
it is necessary to have duct sections free from disturbances both before and after the plane of
measurement with length a as in Table 2 below. The table states the minimum requirements of straight
sections and the selected plane of measurement must be checked according to clause 7.1.3.

Table 2: Necessary straight sections before and after plane of measurement

Straight sections Circular duct Rectangular duct
Before plane of measurement
a ≥ 5 ⋅ D a ≥ 6 ⋅ D
h
After plane of measurement
a ≥ 2 ⋅ D a ≥ 2 ⋅ D
h
7.1.2.3 Preparation procedure and selection of measuring planes
1. Select the location of the plane of measurement according to Table 2, taking into consideration the
required straight sections. After certain types of disturbances, among others throttling dampers,
considerably longer straight sections may be required. In the case of circular ducts, the plane of
measurement should be located at least 150 mm upstream of any duct joints. For rectangular ducts,
the plane of measurement should be located at least 50 mm upstream of any C-clip joint. Rectangular
ducts with a dimension exceeding 600 mm are normally cross-profiled in order to be more stable with
regard to pressure changes. Measurements should if possible be made from a non-profiled side. Note
that the distance between the plane of measurement and following flow obstacles must be at least 2 D
(or in the case of rectangular ducts at least 2 D , the hydraulic diameter).
h
2. Remove the external insulation at the point of measurement. Avoid measurements in internally
insulated ducts since it is difficult to exactly determine the location of the point of measurement. If
measurements are made in internally insulated ducts the diameter must be recalculated, taking into
consideration the thickness of the insulation and the measurement points according to Table 3 or
alternatively Tables 4 to 6 with modification.
Table 3. Measurement points for circular ducts

Diameter Position of a b c d Figure
1)
Nominal D measurement
mm points mm mm mm mm
100 29 71
a=0,29*D
125 36 89
b=0,71*D
160 46 114
200 20 100 180
a=0,10*D
250 25 125 225
b=0,50*D
315 32 158 283
c=0,90*D
400 40 200 360
500 22 145 355 478
a=0,043*D
630 27 185 445 603
b=0,290*D
800 34 230 570 766
c=0,710*D
1000 43 290 710 957
d=0,957*D
1250 54 360 890 1196
1) According to duct standard
Table 4: Measurement points for rectangular ducts on the wider dimension: L width
150 << L ≤≤ 300 mm 300 << L ≤≤ 2000 mm
<< ≤≤ << ≤≤
2 2
a=0,08*L   b=0,43*L a=0,06*L  b=0,235*L  c=0,43*L
2 2 2 2 2
c=0,57*L   d=0,92*L d=0,57*L  e=0,765*L  f=0,94*L
2 2 2 2 2
Table 5 – Measurements points for rectangular ducts on the wider dimension – values recalculated
according to L2
L 150 200 250 300 400 500 600 800 1000 1200 1400 1600 1800 2000
a 13 16 20 25 25 30 35 50 60 70 85 95 110 120
b 65 85 110 130 95 120 140 190 235 280 330 375 420 470
c 85 115 140 170 170 215 260 345 430 515 600 690 775 860
d 137 184 230 275 230 285 340 455 570 685 800 910 1025 1140
e - - - - 305 380 460 610765 920 107012251380 1530
f - - - - 380 470 565 750940 1130131515051690 1880
...